AMPARA, Sri Lanka (AP) _ A bomb hidden in a motorcycle exploded in an eastern Sri Lankan town Wednesday, injuring 18 people, police and witnesses said.

Seventeen others were killed in clashes in northern Sri Lanka and four navy seamen died when a rebel boat rammed their craft off the northeast coast.

The motorcycle carrying the bomb was parked in a central district in Kalmunai town, about 20 miles northeast of Ampara, police said. Ampara, about 125 miles east of the capital of Colombo, is eastern Sri Lanka's main town.

At least four of the wounded were members of the government's anti-terrorist Special Task Force, said a senior police official speaking on customary condition of anonymity. At least seven shops were damaged in the explosion.

No one immediately claimed responsibility, but police routinely blame bomb attacks on separatist rebels of the Liberation Tigers of Tamil Eelam.

Meanwhile, a rebel boat crashed into a navy craft six miles off the coast of the northern Jaffna peninsula and exploded, killing four sailors, navy spokesman Lieutenant Commander Sarath Mohotti said.

It was not clear whether the rebel boat deliberately rammed the navy craft.

Thirteen guerrillas and four soldiers were killed in routine fighting in the northern battle zone, military spokesman Nishantha Wadugodapitiya said in Colombo.

The Tamil Tigers are trying to carve an independent homeland for minority Tamils in the north and east of the country. More than 58,000 people have been killed since the fighting began in 1983 in this small island nation off India's southern coast.